Hepatic leukemia factor (HLF) is a member of the proline and acid rich (PAR) family of transcriptional activator proteins, a subset of the larger superfamily of transcription factors known as the basic region/leucine zipper (bZIP) proteins. HLF was first detected due to a translocation in certain pro-B-cell acute lymphoid leukemias, resulting in the aberrant expression of the chimeric protein, E2A-HLF. It now seems likely that E2A-HLF promotes leukemogenesis by interfering with an early step in an evolutionary conserved pathway leading to cell- death, or apoptosis. It is not presently understood why cells decide to initiate apoptosis, and the study of HLF provides an opportunity to study this question in a vertebrate system. This research proposal addresses the mechanisms of regulation of HLF. The rat HLF gene encodes two transcriptional activators, and possibly a transcriptional repressor. The gene is transcribed from two promoters, an alpha-promoter directing highly ciradian transcripts encoding HLF43, and a less circadian beta-promoter encoding HLF36. To further understand how these alternative HLF proteins modify gene expression, three lines of investigation are proposed: 1) The analysis of HLF N-terminal deletion mutants is proposed to identify amino-acid residues and protein domains involved transcriptional activation. Since the two HLF activators, differing only in the presence (HLF43) or absence (HLF36) of 49 amino acids, have different target preferences, these 49 amino acids may inhibit some promoters and stimulate others. The molecular dissection of of the activation domain proposed here is aimed at understanding the mechanism of this differential activation. 2) The characterization of the promoter elements and trans-acting factors involved in the transcriptional regulation of the HLF gene should identify those components that direct the tissue-specific and circadian expression of the gene. 3) In order to understand where and when HLF is present in the developing embryo, immunohistochemical examination of HLF proteins in tissues throughout development is proposed. Many diseases are associated with defects of regulation of apoptotis, including cancer, autoimmune disorders and viral infections, AIDS, Alzheimer's disease, and ischemic stroke. Understanding of the regulation of apoptotic effector molecules, such as HLF, will increase our knowledge about the pathways leading to cell-death in vertebrates, and thus enhance our ability to remedy such malfunctions.